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Week 14 · Chapter

Chapter 14 — The Peripheral & Autonomic Nervous System

Human Anatomy and Physiology I Free sample edition

The module's primary reading · ~40–50 minutes · Objective 7 (describe the organization of the nervous system — the brain's major regions and their functions, the spinal cord, the meninges, cerebrospinal fluid, and the reflex arc in the central nervous system; the cranial and spinal nerves, afferent and efferent traffic, and the somatic and autonomic divisions in the peripheral nervous system; and the opposing sympathetic and parasympathetic branches as a homeostatic pair), Objective 1 (use the language of the body correctly — the anatomical position, the directional-term pairs, the three body planes, and the body cavities and abdominal regions — distinguish anatomy from physiology, climb the levels of structural organization, list the functions and requirements of human life, and trace a homeostatic feedback loop through its receptor, control center, and effector, telling negative feedback from positive feedback) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the Reading Your Own Autopilot lab all build on it.

In plain language

Last week you met the brain and spinal cord, the body's command center; this week you follow the cables that run from it to everything else. Those cables are nerves (bundles of axons outside the brain and spinal cord), and they carry signals in two directions: in from your senses, and out to your muscles and glands. Some outgoing commands are ones you choose, such as lifting a fork; others run on autopilot (automatic control you never have to think about), such as your heart speeding up when a door slams. You will learn to count the nerves, sort their traffic by direction, and name the two branches of the autopilot that push your body in opposite directions. By the end, a racing pulse that settles after a scare will read as a short, ordered story.

Why you already care about this

Picture three made-up moments. A door slams behind someone at home, and the heart jumps before the person has turned around. A runner in a stairwell stops at the top floor, and the pulse in the wrist is pounding. An hour after dinner, the stomach is churning quietly while the person reads on the couch. Nobody decided any of those changes. Something decided for them, and it used 12 pairs of cranial nerves, 31 pairs of spinal nerves, and 2 opposing branches of an autopilot.

Here is the surprise. The same heart is being pushed in two directions at once, all day long. One set of nerves presses the gas; another set presses the brake. Which one wins at any moment decides whether your heart beats fast or slow. That tug-of-war is not a problem to fix; it is how the body holds itself steady.

By the end of this chapter you will be able to:

name the parts of the peripheral nervous system and count its 12 pairs of cranial nerves and 31 pairs of spinal nerves;
sort a signal as afferent or efferent, and an outgoing command as somatic or autonomic;
name the sympathetic and parasympathetic divisions with their nickname, their origin in the cord, their chemical messenger, and their effects;
explain dual innervation and why the two branches' balance is homeostasis;
average three heart-rate counts, find the rise and the fall, and name the branch behind each move, with every step and unit shown.

Carry this line with you: two pedals, one heart — gas versus brake, never one alone.

Where we are

Already covered: Week 1, Body Organization, Homeostasis & Anatomical Terminology, built the feedback loop: a receptor senses, a control center compares, an effector responds. Week 5, Tissues, named cardiac muscle, smooth muscle, and glands. Week 9, The Skeleton & Joints, counted 7 cervical, 12 thoracic, and 5 lumbar vertebrae. Week 10, Muscle Tissue & the Physiology of Contraction, showed acetylcholine leaving a motor neuron at the neuromuscular junction. Week 12, Nervous Tissue, the Neuron & the Action Potential, showed how a neuron fires and passes a chemical messenger across a synapse. Week 13, The Central Nervous System, mapped the brain and spinal cord and put the reflex arc in order.
This week: the peripheral nervous system's nerves and ganglia, the cranial and spinal nerve counts, afferent and efferent traffic, the somatic and autonomic divisions, and the sympathetic and parasympathetic branches read on a heart-rate table.
How they connect: Week 13's brain and cord make the decisions; this week's nerves carry them out, and the autonomic branches act as the effector side of Week 1's feedback loop.
Where we're going next: Week 15, The Special Senses, follows three of this week's cranial nerves back to the eye, the ear, and the nose, and closes the course's teaching weeks.

Here is what you lean on this week. In Week 13 you learned that the central nervous system (CNS) is the brain and spinal cord, and that the brainstem regulates the heart and breathing. You also learned the reflex arc: receptor, sensory neuron, an interneuron in the cord, motor neuron, effector. In Week 12 you learned that a neuron passes its signal across a synapse with a chemical messenger, and two of those messengers, acetylcholine and norepinephrine, return this week as the signatures of the two autonomic branches.

1. Map the wiring: nerves and ganglia outside the CNS

Think about the wiring in a house. The fuse box is in one place, but cables run out of it to every room, every light, and every outlet. Your nervous system has the same plan. Last week you studied the fuse box. This week you follow the cables.

The peripheral nervous system (PNS) is the ganglia and nerves outside the brain and spinal cord. Two kinds of structure build it.

A nerve is a bundle of axons running outside the brain and spinal cord; nerves carry signals between the central nervous system and the rest of the body.
A ganglion is a cluster of neuron cell bodies that sits outside the brain and spinal cord, part of the peripheral nervous system.

So the cell bodies of the PNS gather in ganglia, and their long axons travel together in nerves. Every signal between the CNS and the rest of the body travels on PNS wiring, whether it is a touch on your fingertip heading in or a command to your heart heading out.

Misconception check. Students often think "a nerve is one long neuron." It is tempting because a nerve looks like a single cord. The cure is the definition: a nerve is a bundle of many axons traveling together, and the cell bodies they belong to sit in the CNS or in ganglia.

2. Count the cranial nerves: 12 pairs and the four to know

Touch your nose, your eyes, and your ears. Signals from all three reach your brain without passing through your spinal cord. They travel on nerves that leave the brain directly.

A cranial nerve is one of the 12 pairs of nerves that leave the brain, numbered CN I through CN XII. The letters CN stand for cranial nerve, and the Roman numeral after them is the nerve's number. This course names four of the twelve:

The olfactory nerve is cranial nerve I, a purely sensory nerve that carries the sense of smell.
The optic nerve is cranial nerve II, a purely sensory nerve that carries vision from the eye.
The vestibulocochlear nerve is cranial nerve VIII, a purely sensory nerve that carries hearing and balance from the inner ear.
The vagus nerve is cranial nerve X, which runs far beyond the head to help regulate the organs of the chest and upper abdomen and carries the parasympathetic calming supply to the heart and gut.

The first three are purely sensory: they carry signals in, never commands out. The vagus nerve is the one to remember for this week. It is a cranial nerve that leaves the head entirely and wanders down to the heart and the gut, and you will meet it again in Sections 7 and 9.

Worked example — sort four cranial nerves. You place each nerve by its number and its job.

CN I, the olfactory nerve: smell, purely sensory.
CN II, the optic nerve: vision, purely sensory.
CN VIII, the vestibulocochlear nerve: hearing and balance, purely sensory.
CN X, the vagus nerve: reaches the organs of the chest and upper abdomen.
In one line: three of the four named nerves carry senses in; the vagus reaches the organs.

Misconception check. Students often misstate the count, writing 31 pairs of cranial nerves or 12 pairs of spinal nerves. It is tempting because both numbers arrive in the same week. The cure is to tie each count to its source: the brain gives off 12 pairs, CN I through CN XII; the spinal cord gives off the other 31.

3. Count the spinal nerves: 31 pairs, every one mixed

Run a finger down the middle of your back. Under it, at every level of the spine, a pair of nerves leaves the spinal cord, one to the left and one to the right. Those are the spinal nerves.

A spinal nerve is one of the 31 pairs of nerves that leave the spinal cord, named for the level where each emerges: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

Worked example — Dataset L, the nerve counts (canonical). You add the spinal nerves by region, then add the cranial nerves.

Cervical, in the neck: 8 pairs.
Thoracic, in the chest region: 12 pairs.
Lumbar, in the lower back: 5 pairs.
Sacral: 5 pairs.
Coccygeal: 1 pair.
Add them: 8 + 12 + 5 + 5 + 1 = 31 pairs of spinal nerves.
Add the cranial nerves: 12 pairs of cranial nerves + 31 pairs of spinal nerves = 43 pairs of nerves in all.

Every spinal nerve is a mixed nerve. A mixed nerve is a nerve that carries both sensory and motor axons in one bundle; every spinal nerve is a mixed nerve. So the same spinal nerve that brings you the feel of a pebble in your shoe also carries the command that lifts your foot.

Misconception check. Students often write "7 cervical nerves," because Week 9 taught 7 cervical vertebrae. The cure is to keep the two counts apart: there are 7 cervical vertebrae but 8 pairs of cervical nerves. The two counts differ, so check which one a question asks for.

4. Read the direction: afferent arrives, efferent exits

Think of an airport. Some flights arrive; others depart. A busy terminal handles both, and the board labels each flight by its direction. Signals in a nerve get the same kind of label.

Afferent describes sensory signals that travel from receptors toward and into the central nervous system; afferent arrives.
Efferent describes motor signals that travel out of the central nervous system to muscles and glands; efferent exits.

The memory hook is in the first letters: afferent arrives, efferent exits. A mixed spinal nerve carries both directions at once, in separate axons. Week 13's reflex arc used both words: the sensory (afferent) neuron carried the signal in, and the motor (efferent) neuron carried the command out.

Worked example — label three signals. You read each signal's direction.

A receptor in your fingertip reports a hot mug to the spinal cord: toward the CNS, so afferent.
The spinal cord sends a command to the muscles of your hand: away from the CNS, so efferent.
A command leaves the CNS for a sweat gland in your skin: out to a gland, so efferent.
In one line: in is afferent, out is efferent.

Misconception check. Students often say "afferent means away," because "a" sounds like "away." It is tempting, and it is backward. The cure is the hook: afferent arrives at the CNS; efferent exits it.

5. Split the output: somatic versus autonomic

Sit at a kitchen table with a plate of food. You pick up your fork, and you meant to. A little later, your stomach starts churning, and you did not mean to. Both are efferent commands leaving the CNS, but they belong to two different divisions.

The somatic nervous system is the part of the efferent nervous system that carries commands to skeletal muscle, running the movements a person chooses to make and somatic reflexes such as the withdrawal reflex.
The autonomic nervous system is the part of the efferent nervous system that carries automatic commands to smooth muscle, cardiac muscle, and glands, mostly to keep internal conditions steady.

The somatic side drives the skeletal muscle you met in Weeks 10 and 11. The autonomic side drives the other two muscle tissues from Week 5, cardiac and smooth, plus the glands. Most of the autonomic system's work keeps internal conditions steady.

Worked example — sort four moments. You decide which division sends each command.

Lifting a fork: skeletal muscle, a movement you choose, so somatic.
A stomach churning after a meal: smooth muscle in the gut wall, automatic, so autonomic.
Blinking on purpose: skeletal muscle, chosen, so somatic.
Sweating on a hot day: a gland, automatic, so autonomic.
In one line: skeletal muscle by choice is somatic; cardiac muscle, smooth muscle, and glands on autopilot are autonomic.

Misconception check. Students often say "the autonomic system moves your skeletal muscles," because "automatic" sounds like any movement you do without thinking, such as walking. The cure is the target tissue. Skeletal muscle is the somatic system's target, even in a somatic reflex; the autonomic system commands cardiac muscle, smooth muscle, and glands.

Pause and think, then choose one. (a) List three things your body did automatically in the last hour, and name the tissue (cardiac muscle, smooth muscle, or a gland) you think carried out each one. (b) Describe one movement you made on purpose and one automatic change you noticed at the same moment, and explain why the two belong to different divisions.

6. Meet the accelerator: the sympathetic division

Imagine walking to your car on a dark night when a dog barks right behind you. In a second your heart pounds, your eyes open wide, and your breathing deepens. Your body has switched into a ready-to-act mode, and one branch of the autonomic system did it.

The sympathetic division is the fight-or-flight branch of the autonomic nervous system; it leaves the thoracic and upper lumbar cord, releases norepinephrine at most targets, and speeds the heart, dilates the pupils, opens the airways, pauses digestion, and triggers sweating.

Three words pin it down.

Fight-or-flight is the nickname for the sympathetic division's set of effects, which readies the body to act in an emergency or during exertion.
Thoracolumbar describes where sympathetic neurons leave the central nervous system: the thoracic and upper lumbar parts of the spinal cord.
Norepinephrine is the neurotransmitter the sympathetic division releases at most of its target organs.

Worked example — read the sympathetic set. You list the effects and say what each one does for a body about to act.

A faster heart: more blood moves each minute.
Dilated pupils: the eyes open wide to the scene.
Opened airways: more air moves in and out.
Paused digestion: the gut's work waits.
Sweating: the skin is ready to shed heat.
In one line: every effect readies the body to act.

Misconception check. Students often think the sympathetic branch is "the stress system," switched off unless something goes wrong. It is tempting because its nickname sounds like an emergency. The cure is its job list: it also speeds your heart during ordinary exertion, such as climbing stairs. It is one half of a pair that is always working.

7. Meet the brake: the parasympathetic division

After a big meal, you sit down and feel your body slow. Your heart settles. Your gut gets to work. This is the other branch, and it does nearly the opposite of the first.

The parasympathetic division is the rest-and-digest branch of the autonomic nervous system; it leaves brainstem nuclei and the sacral cord, releases acetylcholine, and slows the heart, constricts the pupils, and promotes digestion.

Rest-and-digest is the nickname for the parasympathetic division's set of effects, which slows the body down and keeps digestion running.
Craniosacral describes where parasympathetic neurons leave the central nervous system: nuclei in the brainstem and the sacral part of the spinal cord.

Its chemical messenger, acetylcholine, is the one you met in Week 10 at the neuromuscular junction. Its most important nerve is the vagus nerve, CN X, which carries the calming supply to the heart and gut.

Worked example — compare the two origins. You place each branch along the CNS, from the top down.

Brainstem nuclei: parasympathetic, which is why the vagus, a cranial nerve, belongs to it.
Thoracic and upper lumbar cord: sympathetic.
Sacral cord: parasympathetic.
In one line: the parasympathetic branch sits at both ends, cranio- and sacral; the sympathetic branch sits in the middle, thoraco- and lumbar.

Misconception check. Students often say "the parasympathetic branch speeds the heart for exercise." It is tempting because "para" sounds like it might mean "extra." The cure is the nickname: rest and digest slows the heart. The sympathetic branch is the one that speeds it.

8. Balance the pair: dual innervation and homeostasis

Think of driving a car down a long hill. Your foot moves between the gas and the brake, never pressing both to the floor, and the car holds a steady speed. Your heart's rate is held the same way.

Dual innervation is the supply of one organ by both autonomic divisions, so the sympathetic and parasympathetic branches compete at the same effector. Most organs receive both. The two branches form an antagonistic pair, two systems that produce opposite effects on the same organs; the sympathetic and parasympathetic divisions form one, and their balance keeps internal conditions steady.

The two autonomic branches' opposite effects on four shared organs
Organ	Sympathetic division	Parasympathetic division
Heart	faster rate	slower rate
Pupils	dilated	constricted
Airways	opened	not named in this course
Digestion	paused	active

The gas-pedal-and-brake model is a classroom model of the two autonomic branches in which the sympathetic division presses the gas and the parasympathetic division applies the brake; it is a simplification, not a description of every real case. Hold it the way you held the thermostat in Week 1: a picture that fits in one hand.

The heart shows the balance best. The heart's pacemaker is the heart's own rhythm-setting tissue, which on its own would fire at roughly 100 beats per minute and is sped up or slowed down by the autonomic branches. That roughly 100 is a figure you learn in words, never as a quiz answer. At rest the parasympathetic branch holds the rate lower. In exercise the sympathetic branch drives it higher.

Now link it to Week 1. A feedback loop needs an effector that can push a variable in either direction. The two autonomic branches give the heart exactly that, one push each way. Their balance is homeostasis at work.

Honesty note: the table and the model are simplifications. This course names four shared organs and a few effects; the real autonomic wiring has more targets and more detail than the course covers.

Misconception check. Students often treat the two branches as one thing, or say "the autonomic system is separate from homeostasis." Both are tempting because the branches share a name and a home. The cure is the table: the branches do opposite jobs on the same organs, and that opposition is one of homeostasis's main tools.

9. Read the table: Dataset K's rise and fall

Picture yourself in a stairwell with a watch. You count your pulse sitting at the bottom, climb briskly for one minute, count again at the top, then rest and count once more. The numbers tell a story about the two branches, and this section reads it.

Beats per minute is the unit of heart rate: the number of heartbeats counted in 60 seconds, or in 30 seconds and multiplied by 2. A 30-second count × 2 gives the same rate unit.

Worked example — Dataset K, the heart-rate table (illustrative: engineered by the course's discipline pack to land on clean numbers, not measured data). Three counts were taken in each condition. You average each set, find the rise and the fall, and name the branch behind each move.

Dataset K (illustrative): three heart-rate counts in each condition, in beats per minute
Condition	Trial 1	Trial 2	Trial 3
Resting	60	64	62
Right after activity	96	100	98
One minute after	70	74	72

Resting average: 60 + 64 + 62 = 186 beats per minute in total, and 186 ÷ 3 = 62 beats per minute.
Right-after-activity average: 96 + 100 + 98 = 294, and 294 ÷ 3 = 98 beats per minute.
One-minute-after average: 70 + 74 + 72 = 216, and 216 ÷ 3 = 72 beats per minute.
Rise: 98 − 62 = 36 beats per minute, the sympathetic branch speeding the heart.
First-minute fall: 98 − 72 = 26 beats per minute, the parasympathetic branch, carried largely by the vagus, re-engaging.
Recovery check: 72 − 62 = 10 beats per minute still above the resting average, so one minute is not a full return to rest.

The rise is, on a heart-rate table, the after-activity average minus the resting average; on the illustrative table it is 98 − 62 = 36 beats per minute. The first-minute fall is, on a heart-rate table, the after-activity average minus the one-minute-after average; on the illustrative table it is 98 − 72 = 26 beats per minute.

Say it in words: the heart rate rose by 36 beats per minute and fell 26 of them back in one minute, the two branches trading control of one effector. Read the branches as a chain: rest → the parasympathetic branch holds the rate below the pacemaker's own pace; activity → the sympathetic branch drives it up; recovery → the parasympathetic branch brings it back down.

In practice (your tools). In the lab you can put your own counts in a free spreadsheet, one condition per row with the three trials in columns B, C, and D. Type =AVERAGE(B2:D2) in column E to average a row, then subtract one average from another, for example =E3-E2, to get the rise. Check each result against your hand arithmetic.

Caveat: Dataset K's numbers are engineered to land on clean whole numbers. Each average is the table's own value, never a norm, and your own counts in the lab will differ.

Misconception check. Students often read the 72 beats per minute as "back to normal." It is tempting because 72 is much closer to 62 than 98 is. The cure is the recovery check: 72 − 62 = 10 beats per minute still above the resting average, so the brake has not finished its work after one minute.

10. Tell the full story: startled, then settled

A loud bang goes off outside a window, much like the slammed door from the opening of this chapter. In a moment the heart pounds, the pupils widen, and the airways open. The sympathetic set has fired.

Then nothing else happens. The noise was a dropped pan, and the threat is gone. Over the next minutes the parasympathetic branch takes the lead again: the heart slows and digestion restarts. One event, two branches, in order:

The bang: the sympathetic branch fires its set, with a faster heart, wide pupils, and open airways.
The threat passes: the bang turns out to be harmless.
The settle: the parasympathetic branch, through the vagus, slows the heart and restarts digestion.

A slow, deep breath nudges the calming branch. The brainstem runs both breathing and the heart's controls, and the vagus carries the calming signal to the heart. This is a description of how the system is wired, not health advice; this course gives none.

Misconception check. Students often think the calm after a scare is the body "doing nothing." It is tempting because calm feels passive. The cure is the order: the parasympathetic branch actively slows the heart through the vagus. Settling down is a command, not an absence of one.

Pause and think, then choose one. (a) Describe a moment when you were startled and then settled, and put your body's changes in the order this section uses. (b) Explain to a friend, in three sentences, why a heart that speeds up and then slows down is a sign of the two branches working, not a sign of a problem.

11. Read the chart: structure, function, and the clinical connection

In a clinic, a pulse is one of the first numbers written on a chart. It looks like a single fact about the heart. This week it reads as the end of a chain.

Worked example — read a pulse on a chart with the three-step read. You trace the chain from the brain to the number.

Structure: the brainstem, the two autonomic branches, the vagus or the sympathetic fibers, then the heart's pacemaker.
Function: the two branches press the gas and the brake on one pacemaker, so the rate written down is their balance at that moment.
Clinical connection: a pulse on a chart is read as the balance of two branches, and the read stops there. It is never read as a diagnosis, and this course gives no advice about any reading.
In one line: brainstem → branch → nerve → pacemaker → the number on the chart.

That chain is what you will measure on yourself. In the Reading Your Own Autopilot lab you count your own pulse at rest, right after a minute of brisk activity, and one minute later, then read each move as one of the two branches. Your counts are your own values, never a norm to meet.

Limitation: a single pulse reading shows the balance at one moment. It cannot tell you which branch moved more, which is why the lab takes three counts in each condition and compares them.

Misconception check. Students often think a pulse that differs from a classmate's means something is wrong. It is tempting because numbers on a chart look like grades. The cure is this course's rule: every measurement is your own value, read as the balance of two branches, never as a norm to meet.

Key terms

Nerve — A bundle of axons running outside the brain and spinal cord; nerves carry signals between the central nervous system and the rest of the body. (first used: 1. Map the wiring: nerves and ganglia outside the CNS)
Ganglion — A cluster of neuron cell bodies that sits outside the brain and spinal cord, part of the peripheral nervous system. (first used: 1. Map the wiring: nerves and ganglia outside the CNS)
Cranial nerve — One of the 12 pairs of nerves that leave the brain, numbered CN I through CN XII. (first used: 2. Count the cranial nerves: 12 pairs and the four to know)
CN — The abbreviation for a cranial nerve, written before its Roman numeral: CN I is the olfactory nerve and CN X is the vagus nerve. (first used: 2. Count the cranial nerves: 12 pairs and the four to know)
Olfactory nerve — Cranial nerve I, a purely sensory nerve that carries the sense of smell. (first used: 2. Count the cranial nerves: 12 pairs and the four to know)
Optic nerve — Cranial nerve II, a purely sensory nerve that carries vision from the eye. (first used: 2. Count the cranial nerves: 12 pairs and the four to know)
Vestibulocochlear nerve — Cranial nerve VIII, a purely sensory nerve that carries hearing and balance from the inner ear. (first used: 2. Count the cranial nerves: 12 pairs and the four to know)
Vagus nerve — Cranial nerve X, which runs far beyond the head to help regulate the organs of the chest and upper abdomen and carries the parasympathetic calming supply to the heart and gut. (first used: 2. Count the cranial nerves: 12 pairs and the four to know)
Spinal nerve — One of the 31 pairs of nerves that leave the spinal cord, named for the level where each emerges: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. (first used: 3. Count the spinal nerves: 31 pairs, every one mixed)
Mixed nerve — A nerve that carries both sensory and motor axons in one bundle; every spinal nerve is a mixed nerve. (first used: 3. Count the spinal nerves: 31 pairs, every one mixed)
Afferent — Describes sensory signals that travel from receptors toward and into the central nervous system; afferent arrives. (first used: 4. Read the direction: afferent arrives, efferent exits)
Efferent — Describes motor signals that travel out of the central nervous system to muscles and glands; efferent exits. (first used: 4. Read the direction: afferent arrives, efferent exits)
Somatic nervous system — The part of the efferent nervous system that carries commands to skeletal muscle, running the movements a person chooses to make and somatic reflexes such as the withdrawal reflex. (first used: 5. Split the output: somatic versus autonomic)
Autonomic nervous system — The part of the efferent nervous system that carries automatic commands to smooth muscle, cardiac muscle, and glands, mostly to keep internal conditions steady. (first used: 5. Split the output: somatic versus autonomic)
Sympathetic division — The fight-or-flight branch of the autonomic nervous system; it leaves the thoracic and upper lumbar cord, releases norepinephrine at most targets, and speeds the heart, dilates the pupils, opens the airways, pauses digestion, and triggers sweating. (first used: 6. Meet the accelerator: the sympathetic division)
Fight-or-flight — The nickname for the sympathetic division's set of effects, which readies the body to act in an emergency or during exertion. (first used: 6. Meet the accelerator: the sympathetic division)
Thoracolumbar — Describes where sympathetic neurons leave the central nervous system: the thoracic and upper lumbar parts of the spinal cord. (first used: 6. Meet the accelerator: the sympathetic division)
Norepinephrine — The neurotransmitter the sympathetic division releases at most of its target organs. (first used: 6. Meet the accelerator: the sympathetic division)
Parasympathetic division — The rest-and-digest branch of the autonomic nervous system; it leaves brainstem nuclei and the sacral cord, releases acetylcholine, and slows the heart, constricts the pupils, and promotes digestion. (first used: 7. Meet the brake: the parasympathetic division)
Rest-and-digest — The nickname for the parasympathetic division's set of effects, which slows the body down and keeps digestion running. (first used: 7. Meet the brake: the parasympathetic division)
Craniosacral — Describes where parasympathetic neurons leave the central nervous system: nuclei in the brainstem and the sacral part of the spinal cord. (first used: 7. Meet the brake: the parasympathetic division)
Dual innervation — The supply of one organ by both autonomic divisions, so the sympathetic and parasympathetic branches compete at the same effector. (first used: 8. Balance the pair: dual innervation and homeostasis)
Antagonistic pair — Two systems that produce opposite effects on the same organs; the sympathetic and parasympathetic divisions form one, and their balance keeps internal conditions steady. (first used: 8. Balance the pair: dual innervation and homeostasis)
Gas-pedal-and-brake model — A classroom model of the two autonomic branches in which the sympathetic division presses the gas and the parasympathetic division applies the brake; it is a simplification, not a description of every real case. (first used: 8. Balance the pair: dual innervation and homeostasis)
Pacemaker — The heart's own rhythm-setting tissue, which on its own would fire at roughly 100 beats per minute and is sped up or slowed down by the autonomic branches. (first used: 8. Balance the pair: dual innervation and homeostasis)
Beats per minute — The unit of heart rate: the number of heartbeats counted in 60 seconds, or in 30 seconds and multiplied by 2. (first used: 9. Read the table: Dataset K's rise and fall)
Rise — On a heart-rate table, the after-activity average minus the resting average; on the illustrative table it is 98 minus 62, or 36 beats per minute. (first used: 9. Read the table: Dataset K's rise and fall)
First-minute fall — On a heart-rate table, the after-activity average minus the one-minute-after average; on the illustrative table it is 98 minus 72, or 26 beats per minute. (first used: 9. Read the table: Dataset K's rise and fall)

Summary

The PNS is the nerves and ganglia outside the brain and spinal cord.
There are 12 pairs of cranial nerves and 31 pairs of spinal nerves.
The 31 spinal nerves split 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
Every spinal nerve is mixed, carrying sensory and motor axons.
The vagus nerve, CN X, reaches the heart and gut.
Afferent arrives at the CNS; efferent exits it.
Somatic commands skeletal muscle; autonomic commands cardiac muscle, smooth muscle, and glands.
Sympathetic is fight-or-flight, thoracolumbar, norepinephrine at most targets.
Parasympathetic is rest-and-digest, craniosacral, acetylcholine.
The two branches compete at the same organs, and their balance is homeostasis.
On Dataset K, the heart rate rose 36 beats per minute and fell 26 in the first minute.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

In an illustrative table made up for this question, a cyclist's three resting counts are 67, 71, and 69 beats per minute, three counts right after a hill climb are 105, 111, and 108, and three counts one minute later are 81, 85, and 83. Find the three averages, the rise, and the first-minute fall, and name the branch behind the rise and behind the fall.
Sort each command as somatic or autonomic: kicking a ball; the gut moving food along after lunch; the pupils widening when a car horn sounds; waving to a friend.
A signal travels from a pressure receptor in the sole of the foot to the spinal cord, and a command then travels from the spinal cord to a muscle of the leg. Label each signal afferent or efferent, and say what kind of nerve carried both.
A made-up chatbot answer says: "During a sprint, the parasympathetic division speeds the heart through the vagus nerve." Find the error and rewrite the sentence correctly.

Answers (worked reasoning — read after attempting)

Resting: 67 + 71 + 69 = 207, and 207 ÷ 3 = 69 beats per minute. After the climb: 105 + 111 + 108 = 324, and 324 ÷ 3 = 108 beats per minute. One minute later: 81 + 85 + 83 = 249, and 249 ÷ 3 = 83 beats per minute. Rise: 108 − 69 = 39 beats per minute, the sympathetic branch speeding the heart. First-minute fall: 108 − 83 = 25 beats per minute, the parasympathetic branch re-engaging. The rate is still 83 − 69 = 14 beats per minute above rest, so recovery is not complete after one minute.
Kicking a ball: skeletal muscle by choice, so somatic. The gut moving food: smooth muscle on autopilot, so autonomic. The pupils widening at a horn: an automatic sympathetic effect, so autonomic. Waving: skeletal muscle by choice, so somatic. The test each time is the target tissue and whether the move is chosen.
Receptor to spinal cord: toward the CNS, so afferent. Spinal cord to leg muscle: out of the CNS, so efferent. Both traveled in a spinal nerve, which is a mixed nerve carrying sensory and motor axons in one bundle.
The parasympathetic division slows the heart; the sympathetic division speeds it. A correct sentence: "During a sprint, the sympathetic division speeds the heart; afterward, the parasympathetic division, through the vagus nerve, slows it back down." The chatbot swapped the two branches, the week's most common slip.

Next up in this module: the lecture, the slides, the Readings & Resources page, the tutorial, the practice set, and the quiz. Next week the special senses follow three of these cranial nerves to the eye, the ear, and the nose, and trace how each sense organ turns a stimulus into a signal.

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